Received: 15 August 2016
|
Revised: 8 January 2017
|
Accepted: 10 January 2017
DOI: 10.1002/acm2.12055
RADIATION ONCOLOGY PHYSICS
Dosimetric characteristics of LinaTech DMLC H multi leaf collimator: Monte Carlo simulation and experimental study Mikaeil Molazadeh1 | Ahad Zeinali2 | Mostafa Robatjazi1 | Alireza Shirazi1 | Ghazale Geraily1 1 Department of Medical Physics and Biomedical Engineering, Faculty of Medicine, Tehran University of Medical Sciences, Tehran, Iran 2
Department of Medical Physics, Faculty of Medicine, Urmia University of Medical Science, Nazloo Campus, Urmia, Iran Author to whom correspondence should be addressed. Alireza Shirazi E-mail:
[email protected]; Telephone: (+98) 2188973661. Funding information Tehran University of Medical Sciences and Health Services, Grant/Award Number: 28202.
Abstract This study evaluated the basic dosimetric characteristics of a Dynamic Multi Leaf Collimator (DMLC) using a diode detector and film measurements for Intensity Modulated Radiation Therapy Quality Assurance (IMRT QA). The EGSnrc Monte Carlo (MC) simulation system was used for the determination of MLC characteristics. Radiation transmission and abutting leaf leakage relevant to the LinaTech DMLC H were measured using an EDGE detector and EBT3 film. In this study, the BEAMnrc simulation code was used for modeling. The head of Siemens PRIMUS linac (6 MV) with external DMLC H was entered into a BEAMnrc Monte Carlo model using practical dosimetry data. Leaf material density, as well as interleaf and abutting air gaps were determined according to the computed and measured dose profiles. The IMRT QA field was used to evaluate the dose distribution of the simulated DMLC H. According to measurements taken with the EDGE detector and film, the total average measured leakage was 1.60 0.03% and 1.57 0.05%, respectively. For these measurements, abutting leaf transmission was 54.35 1.85% and 53.08 2.05%, respectively. To adapt the simulated leaf dose profiles with measurements, leaf material density, interleaf and abutting air gaps were adjusted to 18 g/cm3, 0.008 cm and 0.108 cm, respectively. Thus, the total average leakage was estimated to be about 1.59 0.02%. The step-and-shoot IMRT was implemented and 94% agreement was achieved between the film and MC, using 3%3 mm gamma criteria. The results of this study showed that the dosimetric characteristics of DMLC H satisfied international standards. PACS
87.55.Gh, 87.55.km, 87.55.Qr, 87.56.Fc KEY WORDS
film dosimetry, IMRT QA, MLC transmission, Monte Carlo simulation, multi leaf collimator
1 | INTRODUCTION
integrated with the rest of the linac hardware, or added on externally. One of the primary goals of the QA in IMRT is to determine
IMRT using photon beams is commonly performed with different
the dosimetric characterization of MLC.1 In IMRT treatments, in
types of Multi Leaf Collimators (MLCs). MLCs can be either
addition to the shielding of vital structures, MLCs are in charge of
---------------------------------------------------------------------------------------------------------------------------------------------------------------------This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2017 The Authors. Journal of Applied Clinical Medical Physics published by Wiley Periodicals, Inc. on behalf of American Association of Physicists in Medicine. J Appl Clin Med Phys 2017; 18:2:113–124
wileyonlinelibrary.com/journal/jacmp
|
113
114
|
MOLAZADEH
ET AL.
modulating the intensity of radiation beams, depending on the depth
and 0.01 MeV, respectively. To increase the number of photons gen-
and type of tumor, and have many applications in the creation of
erated in the target, Directional Bremsstrahlung Splitting (DBS) was
volume dose distribution in the three-dimensional form which is in
used.13 Therefore, to maximize dose and fluence efficiency at 6 MV
accordance with the shape of the tumors. MLC provides all of these
beam energy, NBRSPL (DBS splitting number) was set to 1000. The DBS splitting field radius was equal to the side of the square field to
capabilities in IMRT treatments. The specific features of any kind of MLC depend on the materi-
be defined (square field defined at a certain distance from the tar-
als. Therefore, it is important to determine the dosimetric properties
get). Electron range rejection was also used with the ESAVE parame-
2
of the MLC and its effects on the dose distribution. Clinical conse-
ter, which is the energy threshold required to turn on the range
quences resulting from the incorrect determination of these features
rejection, set to 2 MeV.13,14
2–5
Studies con-
The Monte Carlo simulations were validated in two steps. In the
sidered leaf leakage shares as well as tongue and groove effects in
first step, the Siemens linear accelerator head (in the absence of
dose calculations, especially in IMRT treatments with a long duration
MLC) was simulated and validated according to the practical mea-
of radiation time which results in an increase in leaf transmission
surements. In the second step, MLCs were added and validated
share in delivering an extra dose to the patient.6,7 Therefore, an
according to the dosimetry data.
have been previously reported in IMRT treatments.
accurate determination of the dosimetric characteristics of MLCs using an appropriate dosimetry tool is one of the most important parameters in QA tests, in the field of IMRT treatments.8
2.A.1 | Simulation of the Siemens PRIMUS linac
Gafchromic films with special advantages and capabilities are
Realistic and reliable results are obtained when accurate details
among suitable tools recommended for IMRT QA.9,10 The possibility
based on the manufacturer’s data, are used for the simulation. One
of using it in water and in solid water phantoms, its high spatial res-
of the most important parameters in the accurate modeling of the
olution, the possibility of using it in a wide range of radiation and its
linac is the target of the accelerator.15 Another important parameter
low dependency on energy, makes the film a strong tool in the field
is the simulation of the flattening filter, which is located inside the
of dosimetry.11 The BEAMnrc Monte Carlo code also has significant
primary collimator and is used to flatten the beam at a certain depth.
applications in MLC modeling12 and is widely used for accurate radi-
The average energy of the produced beam depends on the geometry
ation dose calculations.
and the materials used in this piece.16 Hence, the target and flatten-
LinaTech Company has produced two types of DMLCs (DMLC H
ing filter play a significant role in MC simulation results; therefore,
and DMLC M models). The DMLC H model has 102 leaves (51 pairs)
the target equipped with a flattening filter can be named the heart
while the DMLC M model has 54 leaves (27 pairs). These leaves can
of the simulation.
be embedded in any type of linac (Siemens, Varian, Elekta, etc.) and
Other constituent parts of the head, including the parallel plate
can also be used with any treatment planning software (Eclipse, Pin-
ionization chamber and mirror were simulated. Thereafter, two pairs
nacle, CMS, etc.). These external MLCs have the ability to implement
of tungsten jaws which lie in a perpendicular direction to each other
both step-and-shoot and dynamic IMRT techniques.
were modeled. These jaws were used for radiation beam collimation
In this study, several dosimetric properties of the DMLC H multi leaf collimator were measured and evaluated. In addition, to determine certain special characteristics of MLC, Monte Carlo modeling was considered.
in the required field sizes. The geometric shape of the simulated Siemens linac head is shown in Fig. 1. Source number 19 in BEAMnrc Monte Carlo code was used for modeling initial electron beam energy. This source has a monoenergetic beam with two-dimensional distribution of Gaussian intensity.17
2 | MATERIALS AND METHODS 2.A | Monte Carlo modeling In accordance with the manufacturer’s geometry and materials, a
The initial electron beam parameters (energy and radius) were determined in accordance with the method proposed by Sheikh-Bagheri and Rogers.18 According to this method, the energy and size of the electron beam will be determined if a good agreement is found between the simulated and measured data.19,20 Electron beam
6 MV medical linear accelerator (Siemens PRIMUS model) was mod-
energy in the range of 5.8–6.6 MeV and its Full Width at Half Maxi-
eled using the BEAMnrc/EGSnrc (Version V4-r2-4-0) simulation soft-
mum (FWHM) in the range of 0.8–2.2 mm was investigated in steps
ware. All parts of the linac head including the target, primary
0.1 MeV and 0.2 mm, respectively. By reviewing MC results with
collimator and flattening filter, monitor ion chamber, mirror and X-Y
the Percentage Depth Dose (PDD) curves (for determining energy)
jaws (secondary collimators) were modeled using modules provided
and lateral dose profiles (for determining energy and especially
by the code. The VARMLC module was used to simulate the exter-
FWHM), electron beam energy and its radius size were determined.
nal DMLC H. Using the DOSXYZnrc software, dose calculation was
These assessments were performed at different square field sizes
performed in a water phantom.
and at various depths (dmax, 5 and 10 cm).
To reduce the simulation run time and increase the efficiency,
Phase space files were used in the validation process. Phase
variance reduction techniques were used in the simulations. The glo-
space files with different arrangements of energies and FWHMs
bal cut-off energy for electron and photon particles was set to 0.7
were generated at a 100 cm Source Surface Distance (SSD).
MOLAZADEH
|
ET AL.
115
respectively. In the direction of the central axis (CAX), the resolution was 1 mm. Therefore, five kinds of phantoms were defined and utilized in the five radiation fields. Considering the number of photon particles used in the simulation, the Monte Carlo uncertainty was less than 1% (2 SD). The configuration and constituent materials of DMLC H were modeled based on the manufacturer’s data; and the other simulation parameters relevant to DMLC H, including MLC density, Z focus of the leaf sides, Zmin (Z of the top of the MLC), interleaf air gap and abutting leaf gap were investigated. Since the average IMRT beam is about 10 9 10 cm2, this field size was selected to determine MLC leakage and transmission. The standard field size was defined using jaws while the MLC was removed from the radiation field (the MLC field size was 30 9 30 cm2). This field size was defined as the Reference Field Size (RFS). Under these conditions, another field called the Closed MLC (C-MLC) was modeled in which MLCs were closed in the center (x = 0). Also, another field called Blocked MLC (B-MLC) was designed. In B-MLC, leaves on one side of the secondary collimators were closed. With the conditions mentioned above and by changing parameters related to the MLC, such as density, Z focus, Zmin, abutting and interleaf air gaps, the dosimetric specifications of DMLC H were investigated.
2.A.2 | Simulation of the LinaTech DMLC H DMLC H contains three leaf banks with different widths. The inner leaf bank is composed of seven leaf pairs with a width of 3.6 mm and 12 leaf pairs with a width of 4.8 mm. As shown in Fig. 2, internal leaves (19 pairs) are specially arranged beside each other. The external leaf bank is composed of 32 leaf pairs with a width of 6.9 mm. The positions of the leaves are projected on the isocenter F I G . 1 . Graphical view of the simulated Siemens linear accelerator head (6 MV Primus) with LinaTech external DMLC H.
plane. The MLC leaves are placed next to each other in the form of a convex and angled in a way that the divergence of radiation beams are taken into consideration (see Fig. 2). The Z focus parameter
Thereafter, symmetrical field sizes of 3, 5, 10, 15, and 30 cm2 were
shows the leaf tip curve radius along the X axis in the direction of
defined by the jaws. The number of histories in different field sizes
the radiation beam. The leaf tip along the Y axis does not have any
was different. The number of electron particles irradiated to the tar-
curve. The maximum useful field size covered with this kind of MLC
get in the standard size field (10 9 10 cm2) was about 2 9 108. For
is about 30 9 30 cm2 in the isocenter area, and the MLC leaf length
field sizes greater than 10 9 10 cm , about 3 9 10 electron parti-
in the above mentioned area is 27.4 cm. These leaves have the
cles were irradiated.
capability of movement along the X axis and to deliver dose to the
2
8
The generated phase space files in the linac isocenter were used as input data in the DOSXYZnrc code. In field sizes of 3 9 3 cm
tumor, it can be arranged into an irregular shape. Screws at the top
2
and bottom of the leaf have a width less than 1 mm, and their
and 5 9 5 cm2, the surface of the water phantom was irradiated
height is 1 mm. The height of MLC is about 7 cm. The upper edge
with 10 9 109 particles. The particles are photons, electrons, and
of MLC is located at a distance of 47 cm from the linac target. In
positrons but the primary particles are mainly photons. In the stan-
simulations, leaf ends were considered in a circular form with a cur-
dard field size, the number of histories was set to 20 9 10 and the
vature radius of 13 cm. For ease in the movement of adjacent leaves
number of photon particles increased with increase in field size. The
and to eliminate the friction caused by movement of adjacent leaves
global cut-off energy for electron and photon particles was set to
(adjacent leaf pairs), a small air gap called interleaf air gap was con-
0.7 and 0.01 MeV, respectively. NBRSPL was set to 1000. Depend-
sidered in the MLC design. Also, to prevent damage to leaves
ing on the field size, different computational resolutions were con-
located opposite each other (abutting leaf pairs), there is another air
sidered. Computational resolution in low-dose gradient areas (the
gap called the abutting leaf gap. In fact, the leaves were separated
region of 80% profile relative to the central axis) and in regions with
by two types of air gaps which can be moved and controlled inde-
high-dose gradient (penumbra regions), was set to 2 and 1 mm,
pendently by the user. With regard to the properties and certain
9
116
|
MOLAZADEH
ET AL.
F I G . 2 . Arrangement of the DMLC H leaves in YZ plane. Seven pairs of leaves with thin width (green) and also 12 pairs of leaves (magenta) are located in the center of the MLC leaf bank. The remaining 32 pairs of leaves, from 51 leaf pairs, make up the outer part of the DMLC H.
specifications mentioned about this type of MLC, among the existing
particles written in the phase space file was about 31 9 106 parti-
modules in the code of BEAMnrc, the VARMLC component module
cles. In DOSXYZnrc simulation, the number of particles in the C-
was used for DMLC H simulation.
MLC test was 4 9 109 particles. To extract the cross-line profile,
Interleaf leakage, intraleaf transmission and MLC leakage
CAX dose value of the RFS field.
computed dose values in the C-MLC field were normalized to the Different simulation parameters, such as MLC material density, Z focus, interleaf air gap, Zmin, and abutting leaf gap were chosen
Tongue and groove design
according to the measurements. By so doing, the difference between
The irregular MLC pattern was designed for investigating the tongue
simulation and measurement results were obtained. The first three
and groove design. The central leaf pairs (leaf pair 26) against each
parameters were determined using the conditions created in the B-
other at a distance of 1 cm from the central axis of the beam were
MLC mode.
closed. Other leaves on both sides of the central leaf were alter-
The inline profile diagram of the MLC leakage was simulated by
nately opened and closed. Opened leaves were placed outside the
applying different numerical changes in the three parameters (den-
radiation field. This kind of leaf arrangement was named Alternated
sity, Z focus, and interleaf air gap). Thereafter, the results were com-
MLC pattern (A-MLC). The Zmin parameter was obtained using the
pared with the leakage profiles obtained from the Radiochromic film
A-MLC pattern. A 10 9 10 cm2 jaw-defined field was used in this
and diode detector. For this purpose, MLC material density in the
test. Measurements were made using a silicon diode detector in
range of 16–19 g/cm3, Z focus in the range of 50 to +50 cm and
water and film in a solid water phantom (at 5 cm depth and 100 cm
air gap in the range of 0.004–0.03 cm were changed. First, to deter-
SSD). The lateral dose profiles obtained from the EBT3 film, EDGE
mine the air gap between adjacent leaves, the air gap value was set
detector and MC simulation were normalized to 100 (to their maxi-
as 0.004 cm and then gradually increased. By adjusting the interleaf
mum relative dose). In MC simulation, the Zmin parameter was chan-
air gap in the mentioned range, the peaks and valleys on the inline
ged from 41 to 45 cm with intervals of 0.5 cm. In the BEAMnrc
leakage profile were observed. Then, by adjusting the density and Z
simulation, 25 9 107 particles were used for the A-MLC field. Also,
focus, the best agreement between simulated and experimental dose
in the DOSXYZnrc simulation, the number of histories in the A-MLC
profiles were obtained.
field was set as 3 9 109 particles.
In BEAMnrc simulation, approximately 4 9 10
8
particles were
In all of the experimental measurements and Monte Carlo calcu-
used for the B-MLC field, of which about 31 9 106 particles were
lations, the SSD was set at 100 cm while the depth was set at 5 cm.
registered in the phase space file located in the linac isocenter. Con-
The A-MLC, B-MLC, and C-MLC fields were measured under similar
sidering the number of photons irradiated to the surface of the
experimental conditions in terms of depth and SSD using an EDGE
phantom (4 9 109 particles), the statistical uncertainty for the DOS-
detector in water and EBT3 film in a solid water phantom. Subse-
XYZnrc calculations was less than 2% (1 SD). To extract the inline
quently, to validate the simulated DMLC H, several square fields
leakage profile, dose values in the B-MLC field were normalized to
(opened by MLC) were assessed with the practical measurements.
the CAX dose value of the RFS field.
2.B | Experimental measurements Abutting air gap The abutting air gap parameter was extracted through the C-MLC field. To determine the abutting leaf gap, a simulation was conducted
2.B.1 | Dosimetry via detectors (Semiflex and EDGE)
according to the C-MLC field. Several simulations were performed
To evaluate the results of MC simulation with experimental mea-
with an air gap range of 0.008–0.2 cm. About 4 9 108 particles
surements, practical dosimetry was performed according to the IAEA
were run in BEAMnrc for simulating C-MLC and the number of
TRS 398 protocol in water,21 and the data was collected using a
MOLAZADEH
|
ET AL.
117
Semiflex cylindrical ionization chamber with 0.125 cm3 nominal sen-
using the EBT3 film were performed in a solid water phantom and in
sitive volume (type TN31010, PTW-Freiburg, Germany) in a motor-
the measurements, film dosimetry protocols (AAPM Task Group
ized 3D ScannerTM (model 1230, Sun nuclear Corporation, Florida,
Report 55)22 and technical considerations as recommended by ven-
USA). The SNC Dosimetry Software (version 1.3.2) was used to con-
dors,23 were considered. Films were scanned using a Microtek
trol the motor system in a 3D Scanner. Moreover, this software was
9800XL flatbed scanner in 48-bit RGB (Red-Green-Blue color repre-
used for data collection and processing. In the SNC 3D Scanner,
sentation) format and analyzed using MATLAB software (R2015a
after the detectors were attached to the scanning system, the water
8.5). The red channel of the EBT3 film was used in film dosimetry.
tank which uses water sensors at three different locations in the
Regarding the fact that the least area of the film should be
tank was automatically leveled. Thereafter, using auto-setup proce-
25 cm2,23 film pieces of 5 9 5 cm2 which were cut from the same
dure, the position of the central axis point was automatically (with-
sheet were used to obtain calibration curves. For the identification
out manual setup) determined. In addition, the Sun Nuclear EDGE
and separation of film pieces with similar dimensions from each
TM
(model 1118) was used for measurements related to
other and to determine shorter length than the original sheet of the
A-MLC, B-MLC, C-MLC, and radiation fields smaller than 5 9 5 cm2.
film, all the pieces were numbered by writing numbers on the top
The EDGE detector is a kind of silicon diode detector with an active
left hand corner. Films were placed at a depth of 5 cm away from
volume of 0.000019 cm3. Unlike the Semiflex detector, the EDGE
the solid water phantom, at the SSD of 100 cm. A field size of
detector is oriented horizontally so that the top surface which is
10 9 10 cm2 was set by jaws. According to the stated conditions,
labeled with a crosshair sign perpendicular to the central axis of the
films with 20 different dose levels, including 0, 30, 50, 100, 150,
beam and toward the source is set. After positioning the effective
200, 250, 300, 350, 400, 450, 500, 700, 850, 1000, 1200, 1400,
point of measurement of the EDGE detector (about 0.5 mm below
1600, 1800, and 2000 cGy were irradiated. Also, to provide full
the surface of the water), the probe moves to the desired depth and
backscatter condition, the total thickness of the solid water phantom
scans are taken. The Siemens PRIMUS accelerator and external MLC
below the film was about 15 cm.24
Detector
mounted on the linac head with the above-mentioned equipment are shown in Fig. 3.
The EBT3 films were scanned 48 h after irradiation using the Microtek scan wizard pro V7.26 software. To improve film response and reduce the error (about 9%) caused by the incorrect placement
2.B.2 | EBT3 film dosimetry
of the film on the scanner, it was scanned in the landscape orientation so that the shorter side of the film was placed along the long
Gafchromic EBT3 films (lot #: 04201502, 8 9 10 inch sheets) were
side of the scanner.25 To obtain raw data, the use of any type of fil-
used in practical dosimetry experiments. Practical measurements
ter and image processing tools was avoided. The films were scanned
F I G . 3 . Equipment used for practical dosimeters, which consist of the SNC 3D Scanner, SNC EDGE detector and the PTW Semiflex ionization chamber.
118
|
MOLAZADEH
ET AL.
in the full dynamic range condition, in the transmission mode and in
recommended by the American Association of Physicists in Medicine
48-bit RGB color mode with a spatial resolution of 127 dpi (0.2 mm)
(AAPM) is test No. 4 of AAPM TG-119.28 According to this test, the
and saved in the TIFF file format.
“C” shaped PTV has a length of 8 cm, the inner radius of 1.5 cm,
For the extraction of calibration curve, pieces of film were exposed
and outer radius of 3.7 cm. A cylindrical organ at risk is located
to doses ranging from 0 to 2000 cGy. To reduce the film dose–re-
inside the target and the center is concentric with the center of the
sponse uncertainty and improve the accuracy of the sensitometric cali-
PTV. In this study, to assess the dose distribution calculations done
bration curve, each dose level was repeated three times and the mean
by MC simulations, the above test was modeled as segmental-IMRT
net Optical Density (netOD) was used to obtain the calibration curve.
using DMLC H in the step-and-shoot technique. The obtained
In order to deliver precise doses to the films, the value of the absorbed
results were compared with the EBT3 film dose distribution. The
dose corresponding to each dose level was obtained with a PTW
IMRT treatment planning was performed with the TiGRT V7.2.24
Semiflex chamber of 0.125 cm3 (model 31010) mounted at a depth of
treatment planning system (LinaTech Co., Sunnyvale, CA, USA). In
5 cm within a solid water phantom. All measurements were performed
treatment planning, 35 segments were designed at nine fields with
according to the IAEA TRS 398 protocol.21
gantry angle intervals of 40°. In addition, the accuracy of the dose
To obtain netOD, prior to exposure, the initial OD (ODinitial) of the unirradiated films was calculated using eq. 1. ODinitial ¼ log10
ðPVunexp PVunopacue Þ ðPVunblank PVunopaque Þ
distribution results obtained from TiGRT was compared with the MC simulated results. In all the assessments, an area of 10 9 10 cm2 (with a pixel spac-
(1)
ing of 1 mm) was selected, and thus, approximately 10000 pixels were analyzed using the gamma index. Gamma analysis is a dimen-
where PVunexp, PVunopaque and PVunblank represent the pixel values of
sionless function that simultaneously takes both Dose Difference
the unexposed film, opaque sheet scan and pixel value of the blank
(DD) and Distance-To-Agreement (DTA) criteria into account.29 For
screen, respectively. Finally, after irradiation, the netOD was
gamma analysis, the dose distribution obtained from the Monte Carlo
obtained using eq. 2.
and EBT3 film via MATLAB software was converted to the DICOM-
netOD ¼ log10
ðPVexp PVopacue Þ ODinitial ðPVblank PVopaque Þ
RT DOSE format and analyzed by the c-index dosimetry module of (2)
the VeriSoft/MEPHYSTO software (version 5.1, PTW, Freiburg, Germany). Through the gamma analysis option,29 the dose distributions
where PVexp, PVopaque, and PVblank represent the pixel values of the exposed film, opaque sheet scan and pixel value of the blank screen, respectively. It should be noted that to obtain the netOD, the ODinitial of each piece of film was calculated separately. In fact, a
were evaluated quantitatively and graphically. In quantitative studies, film measurements were chosen as the reference dose distribution and the other dose distributions were compared against it.
generic background was not used. Instead, the ODinitial was used as the background. The experimental dose, fitting dose and the total dose uncertainties were estimated by error propagation as proposed by Devic et al.24 For the analysis of films, an area of 1 cm2 was selected from the central part of the film (50 9 50 pixels).24 The Levenberg–Mar-
3 | RESULTS 3.A | Experimental validation of Monte Carlo modeling
quardt algorithm was used to obtain an appropriate calibration curve
The results of this study showed a good agreement between the
and minimize the fitting uncertainty.26,27 Finally, the calibration curve
simulated and experimental data when the combination of 6.2 MeV
was obtained by fitting a third-degree polynomial curve.
and 0.09 mm for the incident electron beam parameters were con-
In A-MLC, B-MLC, C-MLC, and RFS fields, films were cut into
sidered. In the build-up area, discrepancies of about 4% were
4 9 25 cm2 and then in accordance with the intended position, they
observed between PDD curves of the MC simulation and measure-
were placed in a solid water phantom. For B-MLC 11500 MU (Monitor
ments, and this discrepancy was estimated to be less than 2% in the
Unit), C-MLC 1000 MU, A-MLC, and RFS field 300 MU were exposed
area after the maximum dose. In the flat area of the lateral dose pro-
to films. For square fields (with and without MLC), the films were irradi-
files, the amount of DD between MC results and practical measure-
ated with 300 MU. Also, the required MU for measuring MLC leakage
ments was about 2% and for all field sizes, the value of DTA in
(in B-MLC field) using the EDGE detector was less than 1000.
areas with high-dose gradient was less than 1 mm. PDD and profile
All radiations were carried out by Siemens Primus linac (6 MV)
curves related to MC simulation (jaw-defined open fields) and practi-
and before irradiation; output was tuned to 1 cGy/MU. The spatial
cal measurements are shown in Fig. 4. Beam profiles and PDD
resolution of MC calculations and detector readings was 1 mm.
curves (defined by MLC) relevant to the EDGE detector and MC simulation for the 6 MV photon beam at a depth of 5 cm are shown
2.C | IMRT QA field
in Fig. 5. The most acceptable agreements for density with a value
The QA tests are one of the basic tests required to commission dif-
equals 0.008 cm. Table 1 shows a summary of the results of
ferent computing systems in radiotherapy centers.5 One of the tests
changes in various parameters.
of 18 g/cm3, for Z focus equals 10 cm while the interleaf air gap
MOLAZADEH
|
ET AL.
119
F I G . 4 . Validation of the MC modeling of the linac head (without MLC) using practical dosimetry: (a) percentage depth dose profiles of the 3 9 3, 5 9 5, 10 9 10, 15 9 15, and 30 9 30 cm2 field size; (b) lateral dose distribution profiles for the same field sizes at a depth of 5 cm. The maximum dose in the current dose distribution profiles was normalized to 60, 70, 80 90 and 100, respectively. Continuous blue lines are the measurements of the EBT3 Gafchromic film whereas the red and yellow squares indicate the results of MC simulation.
F I G . 5 . Comparison of the dose distribution curves along the central axis of the beam (a) and perpendicular to the central axis of the beam (b) of the external DMLC H. The square fields of 3, 5, 10, 15 and 30 9 30 cm2 normalized to 60, 70, 80, 90, and 100, respectively. The continuous lines (blue) correspond to the EDGE detector readouts and the squares (red and yellow) correspond to the MC simulation.
T A B L E 1 Summary of the MC simulation results as compared to the results of film dosimetry to determine the characteristics of the DMLC H by changing various parameters (density, interleaf air gap and Z focus). No.
Material Density (g/cm3)
Interleaf air gap (cm)
Z focus (cm)
1
17.8
0.008
20
2
18
0.008
10
Mean interleaf leakage (mean SD) (D)
Mean intraleaf transmission (mean SD) (D)
Total average leakage (mean SD) (D)
2.03 0.09% (9.73)
1.43 0.03% (5.15)
1.64 0.01% (4.46)
1.74 0.02% (5.95)
1.32 0.08% (2.94)
1.50 0.05% (4.46)
3
18
0.008
0
2.61 0.08% (41.08)
1.60 0.06% (17.65)
1.99 0.10% (26.75)
4
18
0.007
10
1.77 0.06% (4.32)
1.22 0.01% (10.29)
1.54 0.03% (1.91)
5
18
0.008
10
1.95 0.04% (5.41)
1.32 0.05% (2.94)
1.59 0.02% (1.27)
6
18
0.009
10
1.99 0.03% (7.57)
1.39 0.06% (2.21)
1.63 0.01% (3.82)
7
18.2
0.008
10
1.75 0.09% (5.41)
1.25 0.03% (8.09)
1.55 0.08% (1.27)
D= % Difference compared with the results of film.
120
|
MOLAZADEH
EDGE detector, the average interleaf leakage was 1.89 0.11% and
3.B | Dosimetric characteristics of the DMLC H
the average intraleaf transmission was 1.37 0.08%. In film dosime-
3.B.1 | Leakage parameters
try, these parameters were 1.85 0.05% and 1.36 0.09%, respec-
According to measurements carried out using the EDGE detector and
EBT3
film,
the
total
average
ET AL.
measured
leakage
was
1.60 0.03% and 1.57 0.05%, respectively. The average value of abutting leaf leakage obtained using a diode detector and film was 54.35 1.85% and 53.08 2.05%, respectively. Figures 6(a) and 7(a) present the corresponding dose profiles. There is a good agreement between the film and the diode detector data. Although the maximum discrepancy between the dose valleys and peaks was 15.39%, the average difference between the EBT3 film and EDGE detector for leakage was estimated to be about 1.91%. For the
tively. It should be noted that the uncertainty in film dosimetry was less than 5%. The leakage dose distribution of the DMLC H related to MC, EDGE, and EBT3 film are shown in Figs. 6(a) and 6(b). According to the parameters that were determined (density = 18 g/cm3, Z focus = 10 cm and interleaf air gap = 0.008 cm), the average values of total leakage, interleaf leakage and intraleaf transmission were 1.59 0.02%, 1.95 0.04%, and 1.32 0.05%, respectively. The maximum discrepancy between the peaks and valleys of the MC simulation and EDGE detector was 17.38%. This difference between the film and MC simulation was 15.88%.
F I G . 6 . (a) Leakage profile of EBT3 film and comparison with the transmission profile as measured using the EDGE detector and (b) leakage dose distribution predicted by BEAMnrc. MLC leakage profiles were obtained along the Y axis (as shown in the left diagram placed in the upper right corner). The results normalized to standard open field (10 9 10 cm2). According to Monte Carlo calculations in B-MLC field, leaf density and interleaf air gap values were estimated to be 18 g/cm3 and 0.008 cm, respectively.
F I G . 7 . (a) Determination of the amount of leakage caused by the abutting air gap between the leaves via film dosimetry and diode detector measurements. (b) Monte Carlo calculations indicate a 52.80% leakage by taking 0.108 cm air gap between the leaves. According to the left graph in the upper right corner, dose distribution of the abutting air gap was obtained along the X axis and all results were normalized to the opened standard field. Yellow square indicates the field size is 10 9 10 cm2.
MOLAZADEH
|
ET AL.
121
6.28 0.38% and between the MC simulation and film is
3.B.2 | Abutting air gap
4.77 0.23%.
In determining abutting air gap, a good agreement was observed with 0.108 cm. Thus, based on the Monte Carlo simulation, the amount of abutting leaf leakage was 52.80 0.06%. According to
3.C | IMRT QA field
these results, the leakage discrepancy for abutting air gap between
The planar dose distributions between the planned and actual dose
the MC simulation and EDGE detector was 2.85% and between
distributions were assessed using gamma function. The gamma test
the MC simulation and EBT3 film, it was 0.53%. It is evident that
results for different criteria are provided in Table 3. Figure 9(a)
the presence of abutting air gap causes an increase in approximately
shows the gamma dose map between MC and the film with 3%-
53% more than the prescribed dose. Figures 7(a) and 7(b) show the
3 mm gamma criteria. The results of the gamma analysis using 3%-
abutting leaf dose profile obtained from the MC simulation, diode
3 mm, 4%-4 mm and 5%-4 mm criteria showed that the agreement
detector and film measurements in the direction of the X axis.
between MC and the film are 94, 98.7, and 99.5%, respectively. Fig-
To better analyze the results, a summary of the dosimetric
ure 9(b) shows the quantitative dose distribution for a “C” shaped MLC field between TiGRT’s Full Scatter Convolution (FSC) algorithm
parameters of DMLC H are presented in Table 2.
and EBT3 film with 3%-3 mm gamma criteria. According to the above-mentioned gamma indices, 92.1, 98, and 99.1% of the pixels
3.B.3 | Tongue and groove design
passed the gamma analysis, respectively.
The impact of the tongue and groove design in DMLC H modeling was assessed. The dose profiles associated with this test are illustrated in Figs. 8(a) and 8(c). By choosing Zmin equal to 44 cm, MC
4 | DISCUSSION
dose calculations demonstrated a good agreement with the measurements. The outcome of this study showed that the average discrep-
In this study, the dosimetric properties of the LinaTech DMLC H
ancy
were determined using experimental measurements and Monte Carlo
between
the
MC
simulation
and
diode
detector
is
T A B L E 2 Dosimetric characteristics of LinaTech DMLC H multi leaf collimator. The EDGE and Monte Carlo results were compared with the film results. Mean leakage (mean SD)
Interleaf leakage (mean SD)
Intraleaf transmission (mean SD)
Abutting air gap leakage (mean SD)
EBT3 Film
1.57 0.05%
1.85 0.05%
1.36 0.09%
53.08 2.05%
EDGE
1.60 0.03% (D = 1.91)
1.89 0.11% (D = 2.16)
1.37 0.08% (D = 0.74)
54.35 1.85% (D = 2.39)
Monte Carlo
1.59 0.02% (D = 1.27)
1.95 0.04% (D = 5.41)
1.32 0.05% (D = 2.94)
52.80 0.06% (D = 0.53)
D= % Difference compared with the results of film.
F I G . 8 . Assessment of the A-MLC pattern profile related to tongue and groove design: (a) The profiles relating to this pattern using film and diode detector measurements; (b) Tongue and groove design. Yellow square indicates the field size is 10 9 10 cm2. In this MLC pattern position of the two leaves closed in the field at a distance of 1 cm from the central axis of the beam (1 cm off-axis from CAX) and location of the two leaves opened are outside the radiation field; (c) Dose distribution of the tongue and groove pattern that is derived from MC calculations. The results were normalized to 100 (normalized to the maximum dose value multiplied by 100).
|
122
MOLAZADEH
ET AL.
energy and film reading with inexpensive equipment.10,31 In addition,
T A B L E 3 Comparison of pass rates for Monte Carlo simulation and FSC algorithm with various sets of gamma criteria. Two-dimensional dose distribution of the EBT3 film was selected as the reference dose distribution.
films are able to measure planar dose distributions. However, films do not have a suitable response at very low-dose levels because in this range, the film uncertainty (errors of measurement and fitting in
% Fraction of pixels satisfying gamma criteria
calibration curve) is high.9,32 Measurement uncertainties can be due
DD-DTA
3%-3 mm
4%-3 mm
5%-3 mm
4%-4 mm
5%-4 mm
to any of the following reasons: changes in accelerator output in the
Monte Carlo
94
97.6
98.5
98.7
99.5
time required for film exposure, natural uniformities in the sensitive
FSC
92.1
95.8
97.9
98
99.1
layer (sensitive material called crystalline diacetylene monomer) and its thickness in different areas of the film, the possibility of statistical error of the film response at the same exposure dose levels and
simulations. Moreover, the simulated and computed dose distribu-
stochastic variations related to the readout device and conditions of
tions were evaluated as compared to the measured dose distribution
film scanning. Fitting uncertainties are mainly due to the uncertainty
in the complex IMRT plan. The compliance of the measured dose
in the process of curve fitting to the experimental data (netOD-dose
distribution (film) with the calculated dose distributions (MC and
data). In addition, the other interfering factors may be involved, such
FSC) requires accurate determination of the dosimetric parameters
as the dependence of film response to low-energy photons (espe-
of the MLC by an ideal detector. Several studies have evaluated clin-
cially for low-energy scattered photons) and changes in the dose
ical effects related to changes in the dosimetric properties of
rates.10,32 However, by implementing a very strict protocol and
3,4,30
Whenever the accuracy of the instrument used in deter-
attention to technical advices, film dosimetry can achieve excellent
mining these parameters is high, the level of compliance will be high
results. According to the subjects mentioned, the findings of this
and therefore, there will be less dose distribution calculation error.8
study indicate that the EDGE detector in comparison with the film
The advantages and disadvantages of each of the detectors (EDGE
could be an appropriate tool for measuring the dosimetric character-
detector and EBT3 film) are important in determining the interleaf
istics of MLCs.
MLC.
leakage, intraleaf transmission, and total average MLC leakage.
As indicated in Fig. 6, the amount of interleaf leakage in leaves
Measuring MLC leakage using very low MU, quick access to
with a width of 3.6 mm (projected at isocenter) is more as compared
detailed results without need to process the measured data, as well
with leaves with a width of 4.8 mm. In other words, the amount of
as high spatial resolution and precision are the main advantages of
leakage in the central leaves is more than that in the outer leaves
the EDGE detector in determining the dosimetric characteristics of
because; radiation intensity is more reduced by increasing the width
MLC. On the contrary, Gafchromic films have other distinctive fea-
of the leaves. Due to the different arrangement of leaves next to
tures in this regard. These features include excellent spatial resolu-
each other, dose fluctuations in the leakage profile are non-uniform.
tion and sensitivity, independence of the film’s response to the
In addition, the leaf structure, design, and shape of the leaf and
mm
(a)
(b)
40
40
mm Gamma:
1.5 20
20
0
0
1.25 1 0.75
-20
0.5
-20
0.25 -40
-40
0 -40
-20
0
20
40
mm
-40
-20
0
20
40
mm
F I G . 9 . (a) Gamma dose distribution (with 3%-3 mm gamma criteria) related to EBT3 film and MC. The red continuous line corresponds to the 100% isodose line of MC and dashed line corresponds to the film at the same isodose. (b) 2D gamma map of EBT3 film and TiGRT treatment planning system with 3%-3 mm gamma criteria. 100% isodose line of the TiGRT was visualized by the blue continuous line, whereas this isodose line for EBT3 film is black-dashed line.
MOLAZADEH
|
ET AL.
123
energy spectrum can also be affected. Therefore, these factors
in carrying out practical dosimetry and also Farzad Tajdini for pro-
caused the scatter space distribution in the center of the profile to
viding all the specifications required for the LinaThech DMLC H
be higher than other areas and eventually, the amount of transmis-
simulations.
33
sion became more than that of other areas of the profile.
As Table 3 shows, the gamma index pass rate for Monte Carlo calculations is greater than the FSC algorithm. This is because the
CONFLICT OF INTEREST
Monte Carlo simulation uses more accurate physical aspects (taking
The authors declare that they have no conflicts of interest.
into account the interaction of different types of particles such as photons, electrons, and positrons with matter). More importantly, in the Monte Carlo simulation, the detailed geometric and dosimetric properties of the MLC can be properly considered. Nevertheless, several factors may result in differences between the Monte Carlo simulation and experimental measurements. The most important factors include statistical uncertainty in computer calculations carried out by Monte Carlo, mismatch in the geometry and materials of linac head and MLC, as well as systematic34 and random errors in water phantom measurements. Regardless of the foregoing, the Monte Carlo simulation can lead to more accurate results, if the number of particles increases and the voxel size is smaller than the scanning resolution. Of course, this would require a lot of time. However, in the current research, a good agreement was obtained between the simulated and measured dose distribution (IMRT QA field).
5. | CONCLUSION The Siemens HDP 6 MV head, together with the external LinaTech DMLC H were simulated according to the specifications of their manufacturers. The dosimetric specifications of the MLC were determined using diode detector measurements, film dosimetry data and MC simulation. BEAMnrc and DOSXYZnrc user codes were used in the commissioning of DMLC H. A good agreement was observed between the modeled and practical measured data. According to the recommendations of TG-50, the average leaf and interleaf transmission should be less than 2% and these results demonstrated that the leakage characteristics of DMLC H satisfied international standards. To evaluate the accuracy of the MC simulation, especially in the case of the modeling of the DMLC H, the dose distribution of the simulated IMRT field was compared with the EBT3 film dose distribution. The gamma analysis of IMRT QA showed that there is an acceptable agreement MC simulation and the experimental data. Furthermore, it was observed that the FSC algorithm has a suitable capability in dose calculation for IMRT treatments.
ACKNOWLEDGMENTS This research was supported by the vice-chancellor of research at Tehran University of Medical Sciences and Health Services (grant number 28202). The authors acknowledge Omid Hospital administrators, Dr Khashabi and Dr Mehri, for providing the required equipment to implement the practical part of the current project. The authors give special thanks to Kamal Mostafanezhad for help
REFERENCES 1. Boyer A, Biggs P, Galvin J, et al. AAPM Report No. 72: Basic applications of multileaf collimators. Report of Task Group No. 50 Radiation Therapy Committee. Madison (WI): Medical Physics Publishing; 2001. 2. LoSasso T, Chui C-S, Ling CC. Physical and dosimetric aspects of a multileaf collimation system used in the dynamic mode for implementing intensity modulated radiotherapy. Med Phys. 1998;25:1919– 1927. 3. Topolnjak R, Van Der Heide U, Meijer G, Van Asselen B, Raaijmakers C, Lagendijk J. Influence of the linac design on intensity-modulated radiotherapy of head-and-neck plans. Phys Med Biol. 2006;52:169. 4. Klein EE, Low DA. Interleaf leakage for 5 and 10 mm dynamic multileaf collimation systems incorporating patient motion. Med Phys. 2001;28:1703–1710. 5. Klein EE, Hanley J, Bayouth J, et al. Task Group 142 report: Quality assurance of medical acceleratorsa. Med Phys. 2009;36:4197–4212. 6. Deng J, Pawlicki T, Chen Y, Li J, Jiang SB, Ma C. The MLC tongueand-groove effect on IMRT dose distributions. Phys Med Biol. 2001;46:1039. 7. Balog J, Mackie T, Wenman D, Glass M, Fang G, Pearson D. Multileaf collimator interleaf transmission. Med Phys. 1999;26:176–186. 8. Kinsella P, Shields L, McCavana P, McClean B, Langan B. Determination of MLC model parameters for Monaco using commercial diode arrays. J Appl Clin Med Phys. 2016;17:6190. 9. Borca VC, Pasquino M, Russo G, et al. Dosimetric characterization and use of GAFCHROMIC EBT3 film for IMRT dose verification. J Appl Clin Med Phys. 2013;14:158–171. 10. Reinhardt S, Hillbrand M, Wilkens J, Assmann W. Comparison of Gafchromic EBT2 and EBT3 films for clinical photon and proton beams. Med Phys. 2012;39:5257–5262. 11. Butson MJ, Cheung T, Peter K. Weak energy dependence of EBT gafchromic film dose response in the 50 kVp–10 MVp X-ray range. Appl Radiat Isot. 2006;64:60–62. 12. Borges C, Zarza-Moreno M, Heath E, Teixeira N, Vaz P. Monte Carlo modeling and simulations of the High Definition (HD120) micro MLC and validation against measurements for a 6 MV beam. Med Phys. 2012;39:415–423. 13. Kawrakow I, Rogers D, Walters B. Large efficiency improvements in BEAMnrc using directional bremsstrahlung splitting. Med Phys. 2004;31:2883–2898. 14. Joosten A, Bochud F, Baechler S, Levi F, Mirimanoff R, Moeckli R. Variability of a peripheral dose among various linac geometries for second cancer risk assessment. Phys Med Biol. 2011;56:5131. 15. Faddegon BA, Asai M, Perl J, et al. Benchmarking of Monte Carlo simulation of bremsstrahlung from thick targets at radiotherapy energies. Med Phys. 2008;35:4308–4317. 16. Kapur A, Ma C, Boyer A, editors. Monte Carlo simulations for multileaf collimator leaves: Design and dosimetry [abstract]. Chicago, Illinois: World Congress on Medical Physics and Biomedical Engineering July 23–28, 2000. 17. Tzedakis A, Damilakis JE, Mazonakis M, Stratakis J, Varveris H, Gourtsoyiannis N. Influence of initial electron beam parameters on Monte Carlo calculated absorbed dose distributions for radiotherapy photon beams. Med Phys. 2004;31:907–913.
124
|
18. Sheikh-Bagheri D, Rogers D. Sensitivity of megavoltage photon beam Monte Carlo simulations to electron beam and other parameters. Med Phys. 2002;29:379–390. 19. Ma C, Faddegon B, Rogers D, Mackie T. Accurate characterization of Monte Carlo calculated electron beams for radiotherapy. Med Phys. 1997;24:401–416. 20. Antolak JA, Bieda MR, Hogstrom KR. Using Monte Carlo methods to commission electron beams: A feasibility study. Med Phys. 2002;29:771–786. 21. International Atomic Energy Agency. Absorbed dose determination in external beam radiotherapy: An international code of practice for dosimetry based standards of absorbed dose to water. Technical reports series No. 398. Vienna: IAEA; 2001. 22. Niroomand-Rad A, Blackwell CR, Coursey BM, et al. Radiochromic film dosimetry: Recommendations of AAPM radiation therapy committee task group 55. Med Phys. 1998;25:2093–2115. 23. International Specialty Products Advanced Materials (ISP), NJ, USA. Gafchromic EBT3 brochure 2011 [Internrt]. Available from: http:// www.ashland.com/Ashland/Static/Documents/ASI/Advanced%20Ma terials/gafchromic-ebt3.pdf Accessed on 30 April 2016. 24. Devic S, Seuntjens J, Sham E, et al. Precise radiochromic film dosimetry using a flat-bed document scanner. Med Phys. 2005;32:2245–2253. 25. Butson M, Cheung T, Yu P. Scanning orientation effects on Gafchromic EBT film dosimetry. Australas Phys Eng Sci Med. 2006;29: 281–284.
MOLAZADEH
ET AL.
JJ. The Levenberg-Marquardt algorithm: Implementation and 26. More theory. In Numerical Analysis. Berlin Heidelberg: Springer-Verlag; 1978:105–116. 27. Sorriaux J, Kacperek A, Rossomme S, et al. Evaluation of Gafchromicâ EBT3 films characteristics in therapy photon, electron and proton beams. Phys Med. 2013;29:599–606. 28. Ezzell GA, Burmeister JW, Dogan N, et al. IMRT commissioning: Multiple institution planning and dosimetry comparisons, a report from AAPM Task Group 119. Med Phys. 2009;36:5359–5373. 29. Low DA, Harms WB, Mutic S, Purdy JA. A technique for the quantitative evaluation of dose distributions. Med Phys. 1998;25:656–661. 30. Wu QJ, Wang Z, Kirkpatrick JP, et al. Impact of collimator leaf width and treatment technique on stereotactic radiosurgery and radiotherapy plans for intra- and extracranial lesions. Radiat Oncol. 2009;4:1. 31. Fiandra C, Ricardi U, Ragona R, et al. Clinical use of EBT model Gafchromic film in radiotherapy. Med Phys. 2006;33:4314–4319. 32. Arjomandy B, Tailor R, Anand A, et al. Energy dependence and dose response of Gafchromic EBT2 film over a wide range of photon, electron, and proton beam energies. Med Phys. 2010;37:1942–1947. 33. Arnfield MR, Siebers JV, Kim JO, Wu Q, Keall PJ, Mohan R. A method for determining multileaf collimator transmission and scatter for dynamic intensity modulated radiotherapy. Med Phys. 2000;27:2231–2241. 34. Nelms BE, Chan MF, Jarry G, et al. Evaluating IMRT and VMAT dose accuracy: Practical examples of failure to detect systematic errors when applying a commonly used metric and action levels. Med Phys. 2013;40:111722.
